export class Store<K, V> extends Map<K, V> {

  constructor(protected readonly factory: () => V, entries?: readonly (readonly [K, V])[] | null) {
    super(entries);
  }

  public get(key: K, create = true): V {

    if (this.has(key)) {
      return super.get(key)!;
    }

    if (create) {
      const value = this.factory();
      this.set(key, value);
      return value;
    }

    throw new Error('Value not found.');

  }

}